Orthogonal frequency division multiplexing (OFDM) based systems are the most promising candidates for 4th generation broadband mobile communication networks. Such systems deployed with multiple-input multiple-output (MIMO) techniques promise satisfaction of the ever growing demands of multi-media services and applications. OFDM has been successfully used in standards for digital audio broadcasting (DAB), terrestrial digital video broadcasting (DVB-T), and wireless local area networks (WLANs), for example. It reduces receiver complexity in the equalization and symbol decoding stages by transmitting each symbol over a single flat sub-channel. However, OFDM's inability to extract multipath diversity (inherently present in the broadband wireless channel) and to guarantee symbol detection, when channel nulls occur on parallel sub-channels, are two adverse effects associated with its simplicity.
Diversity techniques are proven to be very effective for combating time varying multipath fading in broadband wireless channel environments. In these techniques, some less attenuated replicas of the transmitted signal, either in the time, frequency or spatial domain, or a combination of all three, are provided to the receiver. These replicas minimize the degrading channel imperfections and enhance the system performance. Temporal diversity is widely achieved by using forward error correction (FEC) coding in combination with (random) time interleaving, whereas frequency diversity can be exploited by using non-linear equalizers or Rake receivers in a single carrier system or with a FEC in an OFDM based system. Spatial diversity in the form of spatially separated, cell sectoring, or polarized antennas have also been of major interest in the research community. All of the above mentioned diversity methods are severely dependent on channel scenarios, transmission data rate, Doppler spread and channel delay spread. Therefore, it is very difficult to realize all forms of diversity in one particular system; for example, in case of slow fading channels with large delay spreads, random time interleaving with FEC or channel coding becomes ineffective. Similarly, frequency interleaving becomes useless for channel environments showing a typical frequency-flat profile. In contrast, spatial diversity is the best approach towards mitigating the channel impairments and enhancing performance, as long as signals at the transmit and receive antenna elements are sufficiently de-correlated.
In connection with diversity transmitters, different concepts are being discussed for multi-carrier, in particular OFDM systems. OFDM is ideally suited for broadband frequency selective channels as it gives the opportunity to use the existing transmit diversity techniques (designed for flat fading channels) in such environments. The design and performance criteria for broadband MIMO OFDM systems promises an excellent diversity level, which is multiplicative of transmit and receive antennas, and the number of multipath components of the broadband channel (with an ideal assumption like equal power in all multipath components and fixed delay between them) [1]. However, the orthogonal space-time block code (STBC) processing scheme proposed in [2] and generalized in [3] failed to extract any or almost no multipath diversity in OFDM. Space-time trellis codes (STTC) of [4], promise diversity as well as coding gain but become unattractive due to their complexity in practical realizations. Other transmit diversity techniques like non-orthogonal block codes also faced the same dilemma of lack of frequency or multipath diversity. This gave a research challenge to design new set of codes for OFDM based systems that would extract at least some of the promised advantages of MIMO OFDM.
Nevertheless, some transmitter diversity schemes, in particular Delay Diversity (DD), when modified to be used in OFDM systems gives excellent simplicity and performance. This technique can be found in many forms, where it differs slightly in terms of its placement in the system. Cyclic Delay Diversity (CDD) (a time domain equivalent of Phase Diversity (PD)) is an improved version of DD. In particular, CDD addresses the adverse effects of DD by introducing cyclic time delays instead of simply time delays [5]. For OFDM based systems, CDD is the simplest approach for extracting frequency diversity that itself has no built-in diversity. It converts the spatial diversity into frequency diversity by artificially increasing the channel delay spread. However, it requires an outer channel or a FEC facility to benefit from the induced selectivity.
Over the years, the search for optimal transmit diversity schemes for MIMO OFDM systems led to many transmitter diversity processing structures and configurations. All these proposed schemes tackled the problem of achieving the maximum (spatial plus multipath) diversity and coding gain in frequency selective environments. By trading complexity, additional processing and incorporating pre-coding arrangements, it was shown that theoretical diversity limits could be achieved. The most noticeable diversity transmitters in this regard can be found in [6], [7], [8] and [9].
In [6], a MIMO-OFDM scheme with variable multiplexing gains was presented. This scheme traded data rate for full diversity (spatial and multipath) by employing an arbitrary space-time code (STC), and to achieve maximum spatial diversity OFDM sub-carriers were encoded. On top of this, an outer codec was used for achieving multipath diversity. The amount of frequency diversity is related to the redundancy introduced by this outer codec, making this scheme severely dependent on the outer codec and the number of resolvable multi-paths. Only a fraction of the available frequency diversity could be exploited when considering an affordable rate loss and practical scenarios.
In [7] and [8], linear constellation pre-coding (LCP) based OFDM diversity transmitters were presented. The design of LCP with STC techniques was discussed in [7]. This approach used existing STCs of [3] and [4], and relied on combining these codes with redundant or non-redundant pre-coders. This scheme achieved maximum diversity and coding gain at the expense of spectral efficiency. Another LCP based diversity transmitter was presented in [8]. This scheme did not rely on STC techniques and used digital phase sweeping (DPS) or circular block delay diversity (CBDD), which are the same as CDD or PD. To achieve the full diversity, this scheme was again dependent on the LCP. The design of this LCP has severe implications on realistic channel conditions and restricted the number of diversity braches.
The scheme in [9] used a mapping approach to design full diversity codes from the existing STC techniques for arbitrary power delay profiles, again suffering from severe rate loss for attaining maximum diversity.
Drawbacks of aforementioned techniques include loss of data rate and/or additional transmitter and receiver complexities. Incorporating LCP or some other codecs to extract multipath diversity may not be the best solution. In all wireless and mobile communication systems channel or FEC coding techniques have become an integral part. These techniques can provide a much simpler and cost effective solution in extracting the frequency diversity in broadband scenarios. We have realised that a hybrid of STC schemes and CDD may offer an improved diversity transmitter and method measured in terms of performance, cost and complexity for multi-carrier systems.
WO 03/015334 (Hottinen) discloses a diversity transmitter for use in CDMA systems. The transmitter applies fixed complex weights in the frequency domain to symbols to be transmitted. Hottinen's scheme is not suitable for use in systems that employ OFDM for broadband, as it requires additional processing at the transmitter. Hottinen's scheme would require significant modification to be useful in OFDM systems.